1. Field of the Invention
The present invention relates to ultrasonic transducers such as used for diagnostic imaging, non-destructive material testing and treatment of human organs and to methods for making such transducers.
2. Background of the Invention
Many types of transducers have been developed for a variety of imaging applications. Ultrasonic devices such as single element, annular arrays, one-dimensional arrays, 1.5 dimensional linear arrays and two-dimensional (2D) matrix arrays are examples of devices used as medical transducers. Recently, matrix shaped ultrasonic transducers designed for three-dimensional (3D) imaging capabilities have been introduced into the marketplace. Reference is made, for example, to U.S. Pat. No. 5,732,706 (White et al) which discloses a piezoelectric matrix array transducer connected into an integrated circuit. For a matrix transducer, the active surface is generally square shaped and the elements are arranged in a N by N matrix fashion wherein each transducer is individually addressed so any focal depth can be electronically controlled. As disclosed in U.S. Pat. No. 5,894,646 (Hanafy et al), typical manufacturing and interconnection methods for matrix transducers are based on the extension of existing manufacturing processes developed for one dimensional linear array transducers. However, these manufacturing methods have led to compromises in performance and to complexity in fabrication. Typically, a single flexible circuit or printed circuit board is used to connect the individual elements of a transducer array to a transducer cable. The use of this technique for a matrix array is not practical because the number of elements involved is significantly higher. For example, there may be 64 to 256 elements in a standard transducer, whereas a matrix transducer has up to 10,000 elements and potentially even more. Standard flexible circuits do not have the density required for this number of elements and thicker multilayer printed circuit boards, such as disclosed in the U.S. Pat. No. 5,855,049 (Corbett et al) degrade the performance of the transducer when placed between the piezoelectric material and the backing material.
According to the requirements of ultrasonic imaging, array transducers must exhibit acceptable acoustic performance to enable the system to provide high quality images. In general, matrix transducers must yield a 2D ultrasonic image quality approaching that obtained with linear array transducers, which means that individual transducer elements of the matrix must be designed to operate substantially identically to conventional transducers.
Ultrasonic transducers are designed to operate in a forward direction, meaning that, in medical applications, the ultrasonic transducer is pointed toward the organ to be imaged. As a consequence, such transducers are constructed to enhance sound propagation from the front face thereof and to minimize sound propagation from the back side thereof. The acoustic energy or reflections emanating from the rear face of the transducer is minimized by the use of a backing material.
Referring to FIG. 1, there is shown, in a cross-section, a typical prior art construction of a linear ultrasonic transducer. The transducer comprises the following components: a focusing lens 3, one or more impedance matching layers or members 2, a piezoelectric layer or member 1, an interconnect layer or member 5 and a sound absorbing backing layer or member 4. Typically, interconnect layer 5 is made of, or formed by, a flexible circuit or printed circuit board. A typical matrix array is shown in cross-section in FIG. 2. The matrix array is similar to the standard transducer except for the absence of a focussing lens and a much thicker interconnection layer 6, and includes an impedance matching layer 8, a further matching layer 9, a piezoelectric layer 7 and a sound absorbing backing layer 10. As illustrated in FIGS. 1 and 2, the interconnect layer 5 or 6 is usually placed at the interface between backing layer 4 or 10 and the piezoelectric material 1 or 7.
Numerous techniques of 2D connection have been developed during recent years, but none of these has provided a satisfactory solution to the problems sought to be solved by the present invention. These prior art attempts sometimes include the use of so-called xe2x80x9cvisiblexe2x80x9d multi-layer circuits which dramatically degrade the transducer response from reflections and sometimes use a connecting method so complicated that the resulting transducer is simply too expensive and unreliable to manufacture.
A further patent of interest here is U.S. Pat. No. 5,267,221 (Miller). This patent discloses an acoustic transducer assembly having a one or two dimensional array of transducer elements, an electrical circuit element such as a printed circuit board and a backing block for interconnecting transducer elements to corresponding contacts or traces of the board. Individual contacts for each transducer element are provided on the top and bottom surfaces of the backing block. The backing block comprises acoustic attenuating material having conductors extending therethrough which interconnect each transducer element to a corresponding circuit contact. The conductors are implemented using thin conductors, conducting fibers or foils, and multiple thin conductors or conducting fibers or foils may be used for each transducer element. This method however requires complicated tooling and methods to align the individual conductors. Furthermore the conductors are so thin as to make it difficult to achieve a reliable contact, and the conductors can collapse if excessive force is exerted on the backing.
There exists a need for a new way of interconnecting individual matrix transducer elements so as to provide a high density of interconnects without compromising the acoustic performance of the transducers.
In accordance with a first aspect of the invention, an improved method is provided for making matrix array transducers (as well as other transducers, as described below). This aspect of the invention is particularly concerned with making the backing block or layer of each transducer, i.e., the sound absorbing portion thereof. A second aspect of the invention concerns improved backing layers, and improved transducers including such backing layers, which incorporate constructional features resulting from the methods of the invention and improved transducers.
According to the first aspect of the invention, a method is provided for making a backing layer for a transducer array, the method comprising: providing a conductive grid comprising a plurality of contacts each having a free end and each being joined together by a common base at an end thereof opposite to the free end so that spaces are provided between the free ends of the contacts; placing the grid in a mold; filling the mold with an acoustically absorbent material such that the absorbent material fills the spaces of the grid; curing the material in the mold so as to form a block comprising the cured absorbent material and the grid; releasing the block from the mold; and removing, e.g., by machining, the common base of the grid in the block so as to separate the contacts from one another within the block.
In one preferred embodiment, the grid is provided by cutting into one surface of a plate of conductive material to form the free ends of the contacts while retaining the common base. Advantageously, the contacts are pyramidal in shape and the cutting step comprises using perpendicular passes of a dicing saw to form said pyramidal contacts. The dicing blade can either have an angular cross-section corresponding to the angle of the pyramidal contacts or alternatively the block can be inclined to create the pyramidal shape.
In an alternative preferred embodiment, the grid is formed by an electro-forming or electro-deposition process. Advantageously, the grid is formed using a master mold having a shape matching that of the grid, electro-depositing metal on the grid, and removing the master mold to form the grid. The master mold preferably includes a plurality of protrusions therein of a shape, and arranged in a pattern, matching that of the contacts so that when said master is removed, hollow contacts are formed.
In one implementation of this embodiment, the hollow contacts are filled with acoustically absorbent material, while, in another, the hollow contacts are filed with metal.
Preferably, after the backing material is molded, the common base is removed by machining away the base. Additionally, the method preferably further comprises machining the block at a surface thereof opposite to the base to expose the free ends of the contacts. The result of the machining is a backing layer and, in an advantageous embodiment, a plurality of machined backing layers are stacked to form a stacked backing layer.
In one preferred implementation, a backing layer is produced which is substantially larger in an area than a transducer to which the backing layer is to be applied and the backing layer is subsequently cut so that the area thereof matches that of the transducer.
In accordance with a further embodiment of the first aspect of the invention, a method is provided for making a backing layer for a transducer array, the method comprising: cutting a plate of conductive material to form a plurality of pyramids joined together at a common base and defining spaces therebetween; placing the plate into a mold; filling the mold with an acoustically absorbent material such that the absorbent material fills the spaces between the pyramids; curing the acoustically absorbent material to form a block comprising the absorbent material and the plate; and machining opposite surfaces of the block such that the plurality of pyramids are separated from one another by machining away the common base to form a plurality of contacts at one surface and such that the tops of the pyramids are exposed so as to form a like plurality of contacts at the opposite surface. The cutting step advantageously comprises forming perpendicular V grooves using perpendicular passes of a dicing saw in order to form the pyramids.
In accordance with one embodiment of the second aspect of the invention, a transducer array is provided which comprises: a transducer layer comprising a plurality of piezoelectric elements; an interconnect layer having a plurality of contacts; and a backing layer disposed between the transducer layer and the interconnect layer, the backing layer including a plurality of conductive micro-contacts extending therethrough from a first surface of the backing layer in contact with the transducer layer to a second surface of the backing layer in contact with the interconnect layer such that the plurality of conductive micro-contacts are aligned with the plurality of piezoelectric elements and with the plurality of contacts of the interconnect layer, the plurality of conductive micro-contacts being of a smaller cross-sectional area at the first surface of the backing layer than at the second surface of the backing layer. In an advantageous implementation, a plurality of the backing layers are stacked between the transducer layer and the interconnect layer.
In one preferred embodiment, the micro-contacts are pyramidal in shape, while, in another, the micro-contacts are conical in shape.
According to a further embodiment of the second aspect of the invention, a backing layer for an ultrasonic transducer array is provided, the backing layer comprising: a layer of acoustically absorbent non-conductive material having first and second opposing surfaces; and a plurality of conductive contacts extending through the layer from the first surface to the second surface so that a first end of the contacts is exposed at the first surface and a second, opposite end of the contacts is exposed at the second surface; the exposed first end of each of the contacts having a cross-sectional area smaller than that of the second exposed end of the contacts. As above, in one advantageous implementation, the micro-contacts are conical in shape, while, in another, the micro-contacts are pyramidal in shape.
Further features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.